Added have_qualifications method to Person QuerySets.

This commit adds a new method to Person QuerySets called
have_qualifications. It takes a list of Skill objects and
returns only those Person objects which have ALL of the
listed skills. I also added a usage note to the "match" view.
Merge pull request #3 from rbeagrie/instructor-search-form

Added have_qualifications method to Person QuerySets.